Either the cam lobe has lost its lift profile or the rocker arm ball has worn thru the rocker arm...

Remove the last two rocker arms on the passenger side and check out the area inside the rocker where the ball sits...compare the two seat areas...if they look the same, you have either a really bad lifter that has collapsed internally or a bad cam lobe...equals engine out in my books...

Post pics of the inside seat area of both arms and their respective pivot balls...